The present invention relates to methods for cleaning metallic surfaces using pulsed hydrogen in a vacuum furnace and particularly relates to methods for cleaning the surfaces of turbine components formed of metallic materials, particularly and for example, cobalt-based alloys, stainless steel and mild steels.
Metallic components, for example, turbine components, particularly turbine nozzles formed of cobalt alloys, develop surface contaminants including surface oxides and surface cracks during usage over time and require refurbishing. Before being refurbished, however, the component surfaces must be cleaned to eliminate the contaminants, e.g., surface oxides including oxidation within the cracks which inhibits the repair of cracks and surface distress. Surface oxides in particular prevent the flow of a fresh material, e.g., a filler of activated diffusion healing (ADH) material, at elevated temperatures due to high surface tension. ADH is a hybrid brazing process that relies on the melting and flow of metal-based material into service-induced cracks or onto surfaces that are being dimensionally reestablished. The success of the ADH repair is dependent upon the ability to adequately clean and/or remove the surface contaminants, including oxides.
The metallic surfaces can, of course, be mechanically cleaned, for example, by wire brushing or local burring with carbide cutting tools. Those methods, however, are low-productivity methods requiring substantial manual labor. To improve productivity, a vacuum furnace or retort using hydrogen gas for cleaning the surfaces has been used. Particularly, hydrogen gas, either in a vacuum furnace (partial pressure atmosphere) or in a furnace retort (at atmospheric or slight positive pressure) has been used to clean surface contaminants and oxides from turbine components including those formed of cobalt-based alloys. When using hydrogen gas to clean such surfaces, a chemical reaction occurs at elevated temperatures within the furnace where the hydrogen reacts with the surface oxides or contaminants to form stable compounds or gases that are subsequently removed. Particularly, when using a partial pressure hydrogen vacuum furnace, the typical approach has been to introduce hydrogen gas into the chamber at a specified temperature and maintain a substantially constant pressure on the order of about 500-10000 microns. In the atmospheric or slight positive pressure approach, a constant flow of hydrogen gas through a retort is maintained and held at temperature. Both of these prior methods, however, do not provide dynamic hydrogen gas flow into tight cracks and the hydrogen gas becomes depleted over time, resulting in no further reduction of oxides. As a consequence, the metallic surfaces are not sufficiently cleaned, which thereby inhibits the adherence of a fresh filler of molten metal e.g., using the ADH process.